Find the sum :

(-11) + (-7) + (-3) + 1 +……to 12 terms.

The above sequence is an A.P. with common difference = -7 - (-11) = 4.

We know that,

Sum of n terms of an A.P. is given by,

∴ S_n = $\dfrac{n}{2}$ [2a + (n - 1)d]

⇒ S₁₂ = $\dfrac{12}{2}$ [2(-11) + (12 - 1)4]

= 6.[-22 + (11)4]

= 6.[-22 + 44]

= 6.(22)

= 132.

Hence, S₁₂ = 132.